It is not currently possible to measure movement of a micro electromechanical systems (MEMS) device in-plane, i.e. in a plane of a wafer of the MEMS device. In the development of such MEMS devices it would be desirable to be able to determine dynamic characteristics of the MEMS device in-plane.
Normally a laser Doppler vibrometer (LDV) is used to measure movement of a MEMS device normal to the plane of the wafer. However, the LDV cannot be used to measure movement of the MEMS device in-plane as the LDV cannot measure at an angle to the MEMS device. This is partly due to the fact that the MEMS device has a reasonably smooth upper surface which cannot act as a retroreflector.
Due to the very small size of a MEMS device, retroreflective material, for example, retroreflective tape, cannot be applied to the surface of the MEMS device. The application of retroreflective tape to the MEMS device would significantly increase the weight of the MEMS device and any subsequent measurement of the movement of the MEMS device, whether in-plane or not, would be entirely meaningless.